e - poz 2 - Central Institute of Brackishwater Aquaculture

Ecology and Fisheries Investigations inVembanad LakeBull. No.107 . dl, 2001Central Inlad Capturn ~hh.rk. &rch Institute(Indian Council of Agriafbral Research)6-re-743 101 West Beng.1

Ecology and fisheries Investigations in Vembanad Lake~4k11errel conrurnrd m rhrb Bullrrrn mob no! hr rt-producctl, rn on! formnrrhrru/ rhr pmu.uon d the puhlrhcrCornposcd stThe ProJut Monitoring &DocumentrUon SectionCIFRI. BarrackporePuhl~shcd hy . The Dlrcctor, CIFRI, Bunc :mePrtntnl OIWS. Topam Pnvrn Ltd.812. Ih. Blmh Gubr S mCalcunr.7000 17

The Vembened take on the south-west coast of Indm In Kerala represents andsymbolizes the nature's bountiful and vend gfi of aquatr Wing resources to manklndO~VKKISIY, the lake ecosystem has a hillfe csrrylng capecity and fosters a crxlal rote bothbnbadlv and eanonncaUv The need to orotect omserve end wtvnsllv . . use such ecosvstemsts tmGraiwe and beyood tihe scope of argument' Howaver. over the part several dkdes.varmus anmrOOOgenlc mtertemu, for manlfold purposes especially construction of~hanneannukkm-barrage, had edvate nnpact on the eco8ystm and fisheryIt n mus wmemlh that ltw wment sI8tus of fishew. carrvina caoacltv of the lake andWvmur of map 'iishery n the poi1 barrage phase IS kn&n m &d& lo sug&sl approproaterenmdlsc measures fa recuvenalma the RsW Wlm mn Intentan scmnttst of ClFRl conducled~nveshgabMs on me lake The b& anem& to present the find~ngs ~n a systematic mannerM. SlnhaDlrector

VEMIANAD LAKf a masswe ud vibrant costal -land eaxyrrrm, stretches m om 24000 ha rnarea and contributes m ovn 50% d the Ism1 area d backwarn (Kayalsl rn Kerala State. h4om than20,000 fishermen are didy dependent on dte aquahc rrsourres d this lake k h i owr ~ 7OOO tof fish and shellfish annually L hdmy wealth cay~lws d ovn 150 spec*, rncludrng the przedvariehu such n the pearl spot, mulko, the gunt frrshwakr prawn and the penwid prawns. Thrf~sh and shellhsh landed from Vmbad lake a valued a owr Rs 100 rnill~on annuallyCoastal wetlands are known to be an ~ndrspensable %tat m a van* of biologically andeconomically Important resident and mrgralory aquahc and avian fauna Thew are alx, mqln~zednursery grounds for several spec= d fish and shellfish Moreover, the lnterdepndence of rnanneand the adjoinrng estuarln zones in cmplaron of tk Irk cyck proceru, d lnnurnerable aquaticspecin is amply described In fihq l~terature The bmlqllcal Importam of Vernbanad arid theinterconnected backwaters are m be view4 and assessed in thls context Thl\ 1s all the moreev~&nt slnce the adlolnlng rnanne coastal zone in the Llshadwrp sea continues to be one of themost product~ve fishery zorm In lt~world contribut~ng to about 0 6 millron t of hsh and fetchlngllvelvhmd for 1 5 million fishermenOn the other s~de. 41 of the 44 riven In the State fill Into the backwater system before they emptyInto the *a Thls accounts for 5 % of India's annual freshwambudget Vembanad lake ltwlfrecewe the anflow trm SIX major rivers of the Stdle with a runoff volume of 24000 mill~o n~' peryear Several of the ftsh spec* esnhabrting the lake share bolh the river and ldke envirolimeiilr Intheor vary~ng lik stages The giant rlver prawn Macrobrach~um rerenberg~r for wtioiri theVembanad ecosystem forms a natural abode. is a classical example of this behav~ourNwenheless, over the past several derddes, (he unmtrtcted human ~nterferenct. lor theheterogeneou5 purpores had irrevocable advenc consequences on the enttly of the ldke 1tw1f Theonblaught wa5 on the lncrease during past few decades and rt still continues The mo5t si~nif~cd~iteventuality among these Mlivitles was the comm#rsroning of the Thanneerrnukkom bdtrdg~ d( IOIIthe lake In 1976 T~IS has destroyed (he contlnurly of the ecosystem proh~b~tlng the sharing of itrptiyso~al dnd b~olog~cal ~dentity between both the seclorr Thus, to quote Gopalan el dl 119831 ~neffect an area ol 69 km' of brxfr~shwater ly~ng south of Thanmcrmulrkom had been rrolog~(dllycut off from the rest of backwaters

The lakc had already been reclatd conuderabhl by 1976 to ntu paddy crop, and the Kumnadare4 mcompasrtng a malor share d the Me had grown lnlo a pnnclpal rm bowi d UI. Thebund was creatrd at Thannemnukkom to pmtcct the low-ly~ng p&dy MI ftMl the t&I nhwab~tngrrs, durlng the Now& to May and lo ntu two aopr of & In an area of ova 6000 hasoh of the barrage Howcwr, faulty dewyrng md paml ~mplementatm of the barrage Mbrought with 11 unant~tparcd m~sfomnes The most d~smntble among them was a vlnulctagnatlon of water mass In the southem Kccor md I& dlo~nlng canals and water ways dwng the5UInIne1, resulting rn ser~our pollution and health hazards In the area On the 0th hmd, banagedu~te* wen unable to nesotlate the monsoon rush fnnn the fwr major nven, and as a result, hrewere frrqwnt flood* durlng the monsoon br~nglng enomus mlsery to the people of Kmnad1 here was another appalling wnseqvnce that tmk place ecolq~cally. The Mor h t got cut offdurln~ the summer expec~enced a tranrforrnat~on fnnn the estuarlne phase to dn almost freshwaterphdw rprultlng In a rap~d declrne In fth y~eld and he ihrlnkage In the species spectrum. Therlverrne or the estuarrm spcln that essent~ally reqwred a solourn Into the estuary or vice vemfaced a sharp fall In thalr populatron size The nurlne mtgratory and the resldent estuarlne specteswen r~tr~cted to the no& of the barcage The replacement of rhe estuarlne species of the sector bythe sweetwater 5pr1es war not adequate to cornpcnute the loss to the f~shervThe ruggetloor for dpproprlate relnedlal measures for reluvenattng the f~rherv requ~red sc~entlftcdoc.ummtatlon on the status d hshery as a whole, cnrylng capac~ly of the system and anonformatloo on the behdviour of malor flrhrry ln he sector durlng he port-barraee phase It was Inth~\ context that he Central Inland Capture Frrherles Research Inst~tute took up the challenge ofcharl~ng syskrnatrally the t~me-scale changes In the ecosystem char~terlstlcs the blodlvers~ty and115 va~nal transformat~onc and fl- tranrformat~on lncorporatlng both the pre- and the postbdrr~g~ phd~)

DHCRlPTlON OF THE VEMMNAD W EThe Vembanad lake spanning between latitudes 9' 28' and 10'10' N andlongitudes 76' 13' and 76" 31' E is the biggest among the 30 backwaters in theState. This has an area of about 22750 km' spread in to three districts, viz.,Emakulam in the north, Kottayam in the east and Alappuzha (Alleppey) in thesouth. The westem side is bordered by a narrow belt of land separating the lakefrom the Lakshadwip sea. It extends to a length of 96.5 km from Azhikode in thenonh to Alappuzha town in the south. The lake north of Aroor is situated in theerstwhile Cochin State, and is generally referred to as the Cochin backwaters. Thesouthern portion from Aroor to Alappuzha is more commonly referred to asVemba~d lake and lies in thc erstwhile Travawwe State. In strict terms, due toits continuity, both the water masses together constitute the Vembanad lake. Thelake is nadw and sinuous in the north while much broader to a maxumum of 14.5km in the south. In its length of over 96 km running parallel to the sea fromAzhikode to Alappuzha, the lake is mainly connected to sea only at two pldres,Azhikode and Kochi. Seasonal intrusion of seawater to the lake is also experiencedthrough the Thottappally spillway, situated 20 km south of Alappuzha. A smallconnection controlled through the spillway exists at Andhakaranazhi, in Ch~rthddltaluk, about 25km nbnh of Alappuzha town. It IS a complex system encompassingestuary, lagoons, marshes and mangroves with intricate network of natural andman-made channels.ORIGIN OF THE LAKE: The Vembanad lake is said to have or~g~nated arouncl5000-6000 years ago durlng the post glacial era when most of the world's coastalsystems took shape. However, according to the geolog~sts, the lake analned 1t5present configuration in the 4" century A. D. The system was primarily a marlneenvironment bound by an alluvial bar parallel to the coast line and interrupted bythe Arabian sea at intervals. As a result of the catastrophic deluge that took placein 1341 A. D., parts of Alleppey and Emakulam districts including a number ofislands thus separated into a distinct water body from the sea with connectingchannels at Thottappally, Andhakaranazhi and Cochin. During this period, theriver Periar emptying at Kodungallur took a diversion through Varappuzha ando p d into Cochin channel, giving rise'to a number of islands lying scattered inthe backwaters by the deposition of the alluvium in its course. This transformation

from the orig~nal marine environment into an estuarine environment is evident bythe large quantities of typically marine shells deposited in the Vembanad region.Thcs transformation has been well explained by Gopalan a/., 1983.The Vembanad lake lying in Kuttanad region is a low lying area near the coast ofthe Kerala. It was originally a part of the shallow coastal area of the Arabian sea.Consequent to geological uplift, a shallow bay was formed into which the riversdracnlng from the mountains to the east discharge. The coast was formed by thes~lt deposits carried by these rivers. The bay thus transformed into a brackishwater lagoon extending from Alappuzha in the south to Cochin in the North. In1976 the lagoon was separated into two by the construct~on of theltlanneermukkom barrier. The barrier was constructed to prevent sal~ne water~ntruslon Into the lake during the dry season (summer months) and also to retainthe fresh water from the rivers flowing into the lake. At the time of ~tsconstruction, the barrier was relatively successful in keeping water in Kundnadlresh arid thereby enabling cropping during the summer months. However, thebarrcer has had various adverse ecological.effects, which would be dealt in deta~lSHRINKAGE OF VEMBANAD LAKE: Backwaters have been cont~nuouslywblected to the reclamatcon for various purposes such as agrccultural expansionagrcculture pract~ces, harbour development, urban development and other publ~cand private uses A few hcstor~cal documents such as the Travancore LandRevenue Manual for the year 1834 testcfy that the local Government of the reglonhad encouraged large scale reclamatcon to the kayal lands (Travancore StateManual, 1834) Of the reclamat~on for the agrccultural purposes, paddy cult~vatlonand paddy-cum-shr~mp culture has contr~buted cmmensely to the horlzontdl5hrinkage of the backwater system In 1834 the Vembanad lake had an area ofnearly 365 km'. Accordcngly to P~lla~ and Pan~kkar (1965) dur~ng the early phases,land reclamat~on and flood control work were largely undertaken at the cnctlatlveof prcvatr farmen wcth actlve asststance from the State Government The detdllqon reclamatron of Vembanad lake for agrccultural and aquacultural purpose ISglven In Tablel. About 2,200 ha area have been recla~med tcll the begcnn~ng ofth~s century. Accord~ng to Kur~an (19781, the reclamatron actcvttles were banrred111 1903 as per the Government dcrect~ve so that ct d~d not h~nder Coch~nHarbour's development due to sclt accumulat~on. But later on, an area of 5,200 ha

was reclaimed up to 1931 (KSSP, 1978). Further, 1320 ha in the Southern part ofVembanad region was reclaimed for agricultural purposes. According to Gopalanet a/. (1983) about 5100 ha. of the backwater has been converted into paddy-cumshrimpculture areas till the 1970s. During 1901 to 1985, an additional area of1500 ha was reclaimed by private ownen for agriculture, cottage industry, andhousrng. The bunding and utlisation of the lake for paddycum-shrimp culture andother activitres is being carried out even now In patches. Thus, out of 36500 ha ofbackwaters which existed till the middle of 19" century, only about 23750ha165.1 %) IS remaining now as open waters. Of this, coconut husk retting activity isesttmated to occupy an area of about 500 ha.Table 1 Rulamltm d Vmtunrd *kr ova th yun- - - - --- - - --- -- --- - -Per~od Area (ha) X 01- Purp~ered&med redmed1981.85 23 91by GCDA and Win TomFlann~ng Trust198145 11 734 11 Hou~lng Kdusbasmnut husk remng0 39 Soulhedy exlasm ofWdrtglon nia?d by GCDA0 07 Foc~hocwtt~an &Mlcpmenl0 03 Cahrn drpyafd CIFT, nwthTanker be& etcBesrdes the hortzontal shrrnkage, The lake also has becn expertcnclng drd5lrrdecltne In 11s depth S~ltatron IS a malor factor contrrbuting to gradudl rhallowrngof the backwater It 1s reported by Gopalan et a1 (1983) that the average depth ofVembanad backwater has been reduced from 6.7 m to 4 4 m between the years1920 and 1980 The sector-wrse depth reduction IS presented In Table-2 Theauthors estlmate that by 1985, the water holdtng capacrty of the lake fromArhrkode to Alappuzha has been reduced to 22 83% of what was rn the begtnnrrrg

of this century. The current lwei of depth in the sourhem sector of the lakeob~ed under the present study was 2.2m, a further depletion of 0.51.3m in aspan of 20 years approximating to 86000 ham.Table 2. Reduction in depth range in different Mom of Vcmbarud lakeover a period of 50 yeanSectors-Depth-range (In mrs)South oi Thanneermukkom bund5QYmulU89 3-3.5.Brtwwn Thannerrmukkom bund and Vaikom 8-9 3 -4Betwe-n Valkom and South Paravmr 7-9 4-5B~rwwn 50uth paravmr and Armr 5-6 34Between Arcmr and south 01 Well~ngton Island 7-8 7-8Bolghdn~ to Chrra~ 3-4.5 2-2.5tl11~rd1 10 Mundmbarn 3-6 2.5-4RIVER DISCHARGE IN TO THE LAKEThe rlvers, which d~rectly drain into the Vembanad wetland system are Chalakudy(1 700 km', 130 km), Periyar (5400 km', 244 km) and Muvattupuzha (1 550 km',12 1 km), In the northen sector, and Meenachal(1250 km', 78 km), Manimala (850km', 90 km), Pampa (2250 km', 176 km) and Achancoil(1500 km', 128 km) In thesouthern sector (Fig.1). The basins together cover an area of about 16200 km' outof which 22 % lies in the lowland, 23 % in the midland and the remaining 55 %In the highland. The rivers flow down the highlands over steep slopes of almost60mkm and more, and traverse the~r midland stretches with much flatter slopes ofabut 1 d m.The rivers are estimated to bring in a total of 10074 million m' of water to the lake(Table3). Of this, 66.54 % are brought in during the three months of the southwest monsoon and 11.67 % during the north east monsoon in October. Only 22% of the discharge are spread among the rest 8 months. Practically there ts noseaward flow of river water during t k months.

- - -- -- ---Jm 42 71 1479 1377 MU 75 BO 0 75Fsb 21 52 12 W 0883 U) 35 0 U)Ma 19 65 01 64 31 28 0 31A$' Jot4 U68 0235 2954 107 12 106May 62 96 9225 1257 42 25 210 33 2 09lor 497 75 75963 41928 64563 2332 71 23 16Jui 47340 85833 45306 mi21 25860 25 67A@ 291 36 631 24 32442 537 24 1784 26 17 71% 83 74 28044 6454 90 €4 4W 40 4 96on 22751 44816 21404 282 83 i in 37 11 aNov 24576 36258 16567 161) R) 962 01 9 55Dsc M) t4 18642 33% 12 36 272 42 2 70TOTAL 2067 01 3758.51 1710.94 2511.02 tW73.Y 1W% 2052 37 31 17 14 2503( omp~lcd from the btd obta~ncd from Dcpanment of lrrpbl~on Co~t01 lterdldTHE THANNEERMUKKOM BARRAGECoriscquent to an unanticipated flood in 1949, The then Agrtculture Min~ster ofthe State decided to find a mechanism to prevent the damage to the paddy cropsrri the reclaimed vast fields in Kunanad area encompassing the Vembanad lake. As))art of this strategy, a bund extending to 4500 feet in length with 93 shutters wasplanned at Thanneermukkom, 24 km north of Alappuzha town. This was toprevent the saline water incursion to the 6000 ha area of the low-lying paddyf~elds In the southern sector of the lake so that a second crop of paddy also couldbe raised in these fields. A leading channel was suggested to divert the floodwaters from the four rivers to sea through a spillway at Thonappally, 20 km southof Alleppey town.

The project was implemented partially. Only 62 shutters were erected at the westand east ends d the bamge. The central won was filled with earth. Theshutters were to prevent the salinity incursion to the southern sector during hight~des of November-December months, and were never intended to be closedpermanently. But, in effect, due to practical problems in intermittently openingand closing the shutters, the barrage mid closed permanently for about sixmonths from November-December to April-May. Only since 1992, the period ofclosure was reduced to four months, from December to April.The Thottappally spillway was also constructed, but with out the lead canal. As aresult, the floodwaters from the four rivers still continue to flood the paddy fieldsas before, occasionally with more severity. The Spillway and the barrage timeagain proved to be obstructions to the seaward flow, causing floods.Tidal flow cutoffConsequent to the commissioning of the barrage across the lake in 1976 and itsclosure during December to April end every year, the tidal inflow too remains cutoff to th~ sector. The result is a virtual stagnation of the water mass in the southernsector for virtually four months of the year. This has destroyed the continu~ty of theecosystem prohibiting the sharing of its physical and biological identity betweenboth the sectors. Faulty designing and partial implementation of the barrageproject had brought with it unanticipated misfortunes. The most discerntbleamong them was a virtual stagnation of water mass in the southern sector and itsadloin~ng canals and water ways during the summer, resulting in serious pollutionand health hazards in the area.With respect to the morphological features, the investigation under the presentprojea has revealed the prominent timexale changes in the southern sector of thelake. The ecological transfonnation in the seaor from the estuarine phsse to analmost freshwater phase resulted in a qualitative and quantitative change in thebiota of the severed aquatic environment. The most apparent change was in thefish fauna. The less noticed but significant changes were among the smallercommunities such as plankton and benthic organisms. The water and sediment

properties also reflected the transformation. The following pages dexribe thesetransformations and put forth certain suggestions which may lead to the ecologicalrestoration of the area for the better benefi of the users.ACTION PROGRAMMES COVERED UNDER THE PROJECfAnalys~s of water and sediment for their physicbchemical properties and nutrient5tdtUSQualitative and quantitative distribution of plankton and benthic fauna.Pr~mary productivity estimationsFishery : estimation of species-wise landings; with an emphasis on the pearlspotI troplos suratensis and the river prawn Macrobrachiurn rosenbergiiEvaluation of changes in the ecology and fishery in the southern sector of the ldhrronsequent to the commissioning of the Thanneermukkom barrage

The barrage at Thanneerrnukkom across the 23000 ha lake. Itsplits the lake into two distinct ecosystem causing changes inthe fisheries and environment.The dense growth of aquatic plants in the lake is a commonsight in the lake after the commissioning of the barrage

DEPTHb9m&f#1h10U)r~b2.lmtql986ararhdltummWm01001VERSIlY CHANGES S&icat &mgm n ptmkbn md qumbtabvdy, the sdlne md~ ~ n ! u e o r l s r r O M n g w alolhe y heshwnle M lo* sekn~lybmrr Thueware&mgmkmobenUm~~dmd~coUIANGES IN FISHERYYURP FALL IN FRESHWATERPRAWN FISHERYTbs~nhecouhanW~l(1~bmmfnnabmm~U m ~ t d h e ~ m g n l o r ) . ~ b y h h e s h w r M r ~ ~r n ~ d ( b ~ ~ d ~ W b dTha fshq d he pnt hhwah~ prm axpaisnced a rhsp W~ne due toIhe~d~~nddown~ndlWVdmcgt@bQI~fhqmawn~euWydwb~~ummcinbarraga Thewkholbqmawn~Uehp~banowr300lonmmlhe80sIobssIhnlWhmmnhe9(k

INvISTGATIONS ON THE FISHERY ECOLOGY OT VEMBANAO M -- A REVIEWMost of the research on the ecology and fisheries of the Vembanad lake wereconducted during the preimpoundment phase, i.e., before the commissioning ofthe Thanneermukkom barrage. The Cochin backwaters and its incoming riversconstituttng the northem wing of the Vembanad lake have been the major area ofInterest to the researchers. Due to this, the literature based on studies extendingup to the southern tip of the lake near Alleppey is rather few (Fig.2).An account on the formative history of the Vemba~d lake has been presented byMenon (1913) and Rasalam and Sebastian (1976). Gopalan (1976) has presented acomprehensive account of the history of reclamation and the consequentshrinkage of the backwaters, especially the Vembanad backwaters. Similarly,under the IndaDutch Collaborative Research Project on the Water Balance Studyof the Kunanad Region, various aspects of the ecology and fisheries of Vembanadldke wt?re investigated and reported (Anon., 1989).WATER QUALITY : Several studies have been conducted on the various physlcclchemical aspects of the Cochin estuary in a fisheries perspective. Balakrishnan(1957) was one of the earliest to study the surface salinity of the Ernakulamchannel, attributed to the rapid salinity fluauations,due to the influence of tide.George and Kartha (1963) recorded the surface salinity of Cochin backwaters Inrelation to the tidal regime. Josanto (1971) analysed the bonom salinitycharacteristics and the factors that influence the saltwater penetration in theVembanad lake extending from the Cochin bar mouth to Alappuzha.Hydrological features in relation to salinity distribution were also studied by a fewmore workers. These include the work of Cherian (1967) Qasim and Gopinathan(l969), Ramamirtham et a/. (1986,1987l and Wellershaus (1 971).SEDIMENT AND NUTRIENTS : The nature of sediment and the distribution anddynamics of nutrients like nitrate, nitrate ammonia, phosphate and silicate in theVembanad lake particularly in the Cochin backwater system has been a major areaof research by several authors. Sankaranarayanan and Quasim (1969) andSankaranarayanan and Panampannayil (1979) concentrated their research on the

nutrient regimen in the water and sediment phases of Cochin backwaters inrelation to other environmental features. Murthy and Veerayya (1972) gave adetailed account d the phosphorous content in the sediments of the Vernbanadlake. Eswara Prasad (1982) has described the. sediment features of m e prawnfarming a- adjacent to the Vembanad lake. A similar study in the prawn fieldsin and around Cochin backwaters is also presented by Aravindakshan et a/. (1992).Purandara and Dora (1987) had studied the textural characteristics and organicmatter of the near shore and surface sediment samples of the Vembanad lakebetween Azhikode in the North and Alappuzha in the South. One majorcontribution was by Sarala Devi (1989) and Sarala Devi et a/. (1992) on thetemporal and spatial distribution of the particulate organic carbon and particulatematter in Coch~n backwaters, especially in the lower reaches of the Periar rlver.Seralathan et a/. (1993) analysed and presented their findings on the features of thesediment and the distribution of organic carbon in the Cochln harbour area. .PLANKTON : One major area of ecological investiagtions in the Vembanad lakewas the qualitative and quantitative distribution of the plankton populat~on.Studies have been conducted on the variation and distribution of phytoplanktonand the factors affecting its production. One of the earliest report In this contextwas of George (1958) who has given an account of the general.composrtlon of theolankton from the Cochin backwaters. Subseauent important contribut~ons were;hose of Qasim and Reddy (1967) on the conientratio" of chlorophyll, Qas~m etal. (1969) on the organic production, Qasim et al. (1974) on the contribut~on ofmicroplankton and the nannoplankton, Gopinathan (1972) on the planktonbiomass, Gopinathan et a/. (1974), and Joseph and Pilla~ (1975) on the total cellcounts and the spatial and temporal distribution of the phytoplankton, .Quantitative and qualitative cmpos~tion of plankton In Vembanad lake extend~ngfrom Cochin to Alappuzha has been investigated by several authors (Devassy andCopmathan 1970, Devassay and Bhattathiri 1974, Kumaran and Raz 1975,Gopalakrlshnan et a/. 1988). The phytoplankton were exclusively studled byGopinathan (1972) in the Cochin backwaters where he reported the presence of62 species of Bacillariophyceae, 24 species of Dinophyceae, 3 specles ofMyxophyceae and 2 species of Cilioflagellates.

Several literatures are available on the distribution and diversity of benthic fauna inthe Vembanad lake during the prebanage phase, and that too concentrating onthe Cochin area. Some d the earlier works in this dimtion include that of Ansari(1974, 19773, Desai and Krishnankutty (19671, Covindankutty (1972), Jayasree(19711, Kurian (1967,1971, 19721, and Remani (1 979). The notable contributionsduring the post barrage phase were by Aravindakshan et a/. (1992), Gopalan et a/.(1987) and Nair and Sarala Devi (1986). Sarala Devi et a/. (1991) elaborated thecoexistence of different benthic communities in the northern limb of Cochinbackwaters. Nair et a/. (1983) gave an account on the population dynamics ofamphipods in Cochin backwater area. Devassy and Copinathan (19701, Kurian etal (1975) and Gopalan et al (1987) undertook some of the investigations on thebenthic fauna extending right from Cochin to Alappuzha.PRIMARY PRODUCTION : Stud~es on the prtmary product~on and 11s relat~onw~th other env~ronmental parameters In the Cochln backwaters have been a majorarea of research by several workers The Important works In th~s d~rect~on are theesttmat~on of plant puynents by Quas~m and Reddy (1967), the solar red~at~on andrelated aspeas by Quaslm et a1 (1968). In depth stud~es on the gross and netproductlv~h/ by l~ght and dark bottle method as well as C" methods wereundertaken by Quas~m et a/ (1969) Th~s study has brought out th~ product~onresplratlon and ass~m~latlon rates In relat~on to the env~ronmental parameten, thrsolar rad~atlon, chlorophyll contents, and the blologlcal componentsPOLLUTION : Sarala Dev~ et a1 (1979), Unn~than ef a1 (1975) and V~jayan el a/(1976) documented the effect of organlc pollution due to ~ndustrlal pollut~on onsome water qual~ty paramers In Coch~n backwater. Reman1 el dl (1980) studiedthe qual~ty of sedlment cn Coch~n backwaters In relat~on to pollut~on aspectsSarala Dev~ et a1 (1986) conducted elaborate stud16 In the Coch~n backwater onthe benth~ communlttes under the Impact of lndustr~al pollut~onFISHERY: Investigations on the distribution and abundance of fishes of Vembanadldke extending from Cochin to Alappuzha have been done by a number ofworkers. Pilla~ (1960) made a record of the distribution of the Hilsa rlrsha In thelake while George (1965) reported the unusual occurrence of mackerel. Shetty(1965) made a comprehensive description of the fishery practice along w~th alisting of the commercially important fish and prawn species of the Vembanad

lake. Kuttyamma (1980) assessed the distribution and abundance of prawns andprawn larvae in Chin backwaters. Raman (1964.1967) made the firstcontribution on the biology of he Macrobrachiurn menbergii and tried toquanttfy its fishery in the lake. He also delineated the nursery grounds of the M.rosenbergri in the river stretches that feed the lake. Rarnan (1975) also gave anaccount of the biology of Ambassis gymnocophalus from the Vembanad lake andcompared with that of the Pulicat lake in Tamil Nadu. Kuttyamma (1980) assessedthe distribution and abundance of pawns and prawn larvae in Cochin backwaters.The fishery estimation during the post barrage phase was mainly by Kurup andSamuel (1980a, 19W, 1983, 1985, 1987). Copinath (1953), Shetty (1965)Kurlan & Sabastian (1982) and Kurup et a/. (1990). Enumeration of the gear andflsh landings was done by them category-wise, sector-wise and species-wise.Kurup and Samuel (1987) listed 150 species of fishes belonging to 100 generacategorlsed under 56 families. The impact of indiscriminate fishery pract~ce andenvironmental stress on t k Macrobrachiurn fishery of the lake has been describedby Kurup (1996). The fishery and biology of four species of Macrobrchium viz.,the M.rosenbergii, M. idella, M.scabriculum, M.equidens In the lake weredescribed by him.The clam fishery has been another important resource of the lake. Its fishery wasstudied by Achary (1987), Kurup et a1 (1990) and Rasalam and Sebastian (1976).The black clam, Villorita cyprinoides formed the major molluscan fishery in theVembanad lake.The survey of the literature reveals that most work on the lake are available for theprebarrage phase and that too restricted to the Cochin sector of the Lake. Theinformation on the southern sector during the post barrage phase is scanty exceptfor the contribution on the fishery by Kurup (1996) and Kurup and Samuel (1980,1987). The ecology stressing on the faunal transformation consequent to thebarrage required more investigation to present a comprehensive change thatoccurred due to the barrage. The Report tries to compensate this lacuna to areasonable extent.

ECOLOGICAL INVESTffiATtONS WRING 1994-97Observations on the water quality parameters indicated a definite pattern ofvariation inflwnced by a combination of saline water intrusion through thebarrage, and the extent of discharge by the four rivers into the lake south of thebarrage CTable-4). The salinity remained the mast important parameter influencingthe general envinmment of the lake. Prior to the construction of the barrage, thetides of a mixed nature were felt during summer upto Pulikkezhu, 80 km south ofCochin connection to the sea, where the maximum tidal range was upto 5 cm, andat the Punnamada, the amplitude was to the extent of 22 cm. Uosanto, 1971).Contrary to this, during the post barrage phase, even in summer, the tidal rangewas restricted till Thaneermukkom Barrage at a distance of only 40 km. Anyenhancement in the salinity further south of the barrage during the summer wasdue to the percolation through the damaged sluices of the barrage. The averagepremonsoon salinity at south of the barrage was 18.47 ppt during the pre-barrgaephase Uosanto, 1971), but reduced to an average of 2.8 ppt during the post-barragephase. Comparatively higher salinity was observed at the western s~de of the lakecaused by the coriolis force operating in the estuary and also due to the absence ofany major freshwater inflow at this bank. The average salin~ty was 0.16 ppt durlngmonsoon and 0.49 ppt during post monsoon season under the present~nvestigation.Temperature was 29.2-32.8 OC ~ndicating a high heat budget for the system. Theshallow nature of the sector (average: depth,1.95 m) coupled with moderate secchidlsc transparency (average:1.26 m) brought almost the entire water column underthe euphotic zone. The average pH of water was near neutral (6.8). During thepre-monsoon stagnation period, the pH was acidic (pH: 6.4; range: 4.5-6.88)part~cularly at stations 2, 4, 7 and 11 (Table 5 & Fig. 3).The d~ssolved oxygen levels were moderate (with a monthly variation rangcng from6.0 to 7.2, average: 6.25 m%L) at all the stations except at Station-1 wheremun~c~pal d~scharge waters brought down the dissolved oxygen to 3.16 mg1Lduring the pre-monsoon period. The Chemical oxygen demand was alsomoderate to h~gh (4.0-28.0 mg/L), again the exceptionally high value recorded atstation-1 during the stagnant pre-monsoon period. The lake in general wascharacterised by low alkalinity (range : 4.0-156.0; average: 22.2 mgfL) except forstatlon 1 and 2.

The nutrients were highly influenced by dre monsoon discharges from the fourriven. The values (available nitrogen: 2C710, average: 213 pgk; availablephospho-rous: Tr.-72, average: 19 pgh) gradually declined during the postmonsoonand pre-monsoon months. The zones receiving the municipal discharge(Station-1) and runoff from agricultural lands and human inhabitation were r~cherIn phosphates and nitrates. The reduction in available phosphates could mainly beattributed to its loss to he sediment in the form of aluminium and iron-bound --- -phosphates at a lower pH. . Silicates had rich concentrations varying from 34.5 to47.0 mg/L (average: 38.5 mg/L).Sediment QualityThe temperature, pH, conductivity, organic carbon, available phosphorus, calciumcarbonate and texture properties of the sediinent were analysed (See Table 4 & 5).The pH of the sediment remained acidic in most of the stations during the studywith an average value of 5.3. The monsoon values were low (average: 4.7). Th~scould be due to the exposure of relatively higher acidic subsurface basin sodconsequent to monsoon turbulence and bottom currents. Available phosphate (av.34.2 mg/100gl in the sediment was high during the premonsoon consequent tothe phosphorous adsorption from the water column. The augmentation was alsoaided by the enrichment from the settled plankton and the subsequentphosphorous release through the microbial activities. The phosphate values forthe sediment phase had an invesre relationship with that of the water phase duringthe three seasons.The organic carbon content had an average value of 2.7% during 1996-97.Stattons 1-4 at the southern end of the lake had elevated levels of organic carb~on(3.27-5.49 %) compared to the 2.8 % reported for this part of the lake by Murthyand Veerayya (1972) during the prebarrage phase. This indicates an accumulationof organic matter in this part over the years. Predictably, the premonsoon seasoncharacterised by the stagnancy indicated higher values of organic carbon at all thestations. Free calcium carbonate was high during the premonsoon followed bythe monsoon and post-monsoon periods.

so.

f8O' +."uns-*"8 n - g.?' o m n r . r.s4--2 z- 0'"' ' 22:;i. ' ;a3."+ . . - 0 . n.""- z* : En--.r*z*..:;;I -r n a 0 * 0 ;* r a r * n- oi., rg:Pe:z- -ndz;'ae2n= a::e- - 0 5 -0 0; z : : ~ ~ : - ; ~ : ~ r r z r . m - -. ----* q0-i 0- 0 ; : " " e m *.O o r n n m* --..,-aoa--n*+..~--. - *e-poz 2I ""- "0 --- r:%%- 2 o r - 0 e " 3---1-::*ro-nnroo-m-,.i - -**..-.." ' - ~ - e t h - - a n o o n r m ~ n n a e o-:-*:--o z g "=:-""O"gN e r - 0-- " .."- u . o - ,8c *0P% ;":$z:s:::fo """ -8"";' 0 9 0 0 z_e "st-;: * -.F ~!f~~~2~;~~~~~~~.~ :::82E: " "Z"P 0 0- w

The sediment texture showed that sand was the major fraction (44.7 %) followedby the clay (31.3 %) and silt (18.7). Studies conductd by Murthy and Veerayyar 1972) during the pre barrage phase showed a silty sand condition, which wasgrddudlly paving the way for a silty clay with the clayey fraction increasing overthe years as observed during the present investigation. Samples collected from theoppo5lte side of the barrage during March, April and October 1996 had theslit + clay fract~on only less than 5 %. This suggests an accummulation of the claydnd silt In the stagnant portion of the lake over the years. Stations 1-3 hads~gnlfitantly high portion of the clay (42.0 %) and silt (22.1%) compared to theother stations lav. 27.8 % and 17.6 % respectively). Stations 1&2 were(omparatlvely less influenced by the monsoon flow (Fig. 4).The study revealed that the water and sediment characteristics showed wides1)atial and temporal differences before and after the construction of the barrage,ttit, differences being more pronounced at the south end of the lake.Plankton Biomass1 he vdr~atlons In the plankton b~omass (senllng volume) with respect to the5f>d50115 and the sampllng stations were determined durcng 1994-96 The preriionsoon perltxl showed the hlghest mean value for the plankton b~omass (4 7n,l/mJ) followed by the post-monsoon (4 4 mllml) and monsoon (34 ml/ml)p~r1Od5 The barrage remains open t~ll early December, the b~omass during thepost monsoon sedson was decided by the outflow to the sea The closure of thebarrage thereafter allowed the environment to be stat~c, and the phytoplanktoncould util~ze the trapped nutrients and flourish Low biomass values observedduring the monsoon period In the present study could be due to the dllut~on of thewater mass and the transport of the b~omass to the sea by the heavy monsoondischarge froni the four rivers The post-monsoon was the recovery season for theblonldss as the plankton could ut~lize nutrients received through the runoff Thedens~ty of organlsrns In the samples presented a more real~st~c and rellable Indexof the abundance than the volume, since the suspended sllt and dirt ~nfluenced theb~omass determlnatlon

The mean biomass varied from 6.6 mVm3 to 0.3 mVml The values were generallyhigh in stations 1,3 and 5. These stations are located nearer to the areas of humansettlement on ttte western fringe. The average highest value was recorded atStation 5 near the denser settlements. At this station, the average biomass was 11mg/L during October to February reflecting the inflow of the nutrients during themonsoon flow. At Station 1, near to the opening end of the city canals to the Lake,the biomass was as high as 13.5 mum'during this period. The canals receive theeffluents from the urban settlements and are known as the worst polluted segmentof the entire Kuttanad region. At the extreme south of the lake, this station had theleast influence of whatever feeble salinty incursion occurred to the Sector. Thisportion remained stagnant through out the summer season. All the three Stat~onsthat depicted high biomass concentration were on the western fringes of the lake,least influenced by the river.flow even during the monsoon season.PhytoplanktonThe net phytoplankton population in the 12 stations of the Vembanad wasinvestigated for a period of two years during 1994-96. At various sampling sltes,phytoplankton had a density variation between 17797 and 60613 unitlm' w~th anaverage value of 30682 unitdmJ. The overall density for the sector was higher lorthe green algae (42.6 %). The blue greens were concentrated at Station-1 w~th 'Ipercentgae contribution upto 70.6 %. Anacystis and Tr~chodesm~urn were ,~lsoabundant at this place, indicating a eutrophic condition. The overall contr~hut~onby the greens and the desmids were prevalent during the monsoon and postmonsoonmonths indicating an improvement in the water condition due to thrmonsoon inflow. The green algae represented by 24 species contributed to thrhigh phytoplankton abundance in the 12 stations of the Vembanad lake. Thr I)rcbmonsoon period (34796 ndmJ) showed a maximum numer~cal abundanctafollowed by the monsoon (32773 no/ml) and the post monsoon (24934 nolm')period (Table 6).Studies conducted during the prebarrage phase by Gopinathan et a/. (1974) In theCochln estuary, a part of the Vembanad Lake, observed a maximum phytoplanktonabundance in the pre-monsoon and monsoon periods. Compared to the prcbarrage phase, the diversity has been found on a higher s~de In the presentinvestigation, ~nfluenced by the prevalence of freshwater condition during the

summer. The pure saline forms that were abundant at this sector during thesummer could not be recorded under the present condition. This obsewatiy is inconformity with the present study. Twenty four species of green algae, tenspecies of blue green algae, one species of yellow brown algae, 13 species .ofdesmids and 19 species of diatoms formed the phytoplankton population in thepresent investigation. But, during the prebamge only about 30 species ofphytoplankton was recorded from the same Sector by Joseph and Pillai (1975)within a salinity range of 0.2 to 33%. Moreover, the species present during themonsoon and post-monsoon period were qualitatively different from that of thepre-barrage phase. There was no perceptible change in the overall density ofplankton population in the sector during the post-barrage phase. The dominantspecies like Hormidium, Spirogyra, Microspora and Anacystis recorded during thepresent study were not reported from the studies conducted during the pre barragephase by Joseph and Pillai (1975).Table.6 Dominant forms of plankton in southern =tor of Vembanad lakebrfon and after the commissioning of thc Th.nccrmukkombmagrPie-barrdge phapostman^, 1971Salinity - 11-19.75 ppt(Nair, rt aL, 1975,joreph & Pillai, 1975)Post-barrage phaseP~monroon 1994-96

The Shannon-Weaver diversity index (HI of the phytoplankton varied from 1.89 atStation 5 to 2.75 at Station 2. A mean diversity index valued 2.43 was recdfor the whole lake. The Station 2 showed the maximum diversity value andrecorded 35 species of phytoplankton whereas station 5 had only 29 speciesrecording lower diversity values. This may be attributed to the comparativelys:aMe environment and a lower fluctuation of salinity at this place. The Stationwas also charmerid by the dominance d the green algae.The zooplankton population in the 12 stations was categorised into 12 groups,namely, Foraminifera, Tintinnida, Protozoa, Rdifera, Nematoda, Polychaetelarvae, Cladocera, Copepoda, Mysidaceae, Ostracoda, lnsecta and Insect larvae,and Fish larvae. The foraminiferans were present only in the marine zone of thelake. The post-monsoon period recorded the highest numerical density ofplankton whereas the monsoon period showed the lowest values. The widevariations in the zobplankton distribution were influenced mainly by the incidenceof phytoplankton, pH and salinity. The present investigation is the firstcomprehensive report on zooplankton from this sector of the Vembanad lake.The premonsoon period, maked by a higher mean salinity, exhibited highernumerical abundance of the zooplankton fauna. As mentioned above, themonsoon and the post-monsoon periods with low saline conditions had restrictedabundance of the zooplankton. Apart from salinity, the strong currentsdownstream and the turbidity also might have rendered an unfriendly environmentto many organisms during the monsoon.Primary produrtivityThe gross and net productivity at stations 1 and 3 in the Vembanad Lake werestudied. The gross productivity values showed an average value of 0.99 g Um'ldayand the net productivity of 0.51 g Um'lday during the period 199697. Qasim eta/. (1969) reported a gross productivity value ranging from 272 to 293 g Um'lyearand the net productivity from 184 to 202 g Umllyear in the Vembanad lake duringthe prebanage phase. Nair et a/. (1975) reported an average primary productivityof 200 mg Um'lday from this sector of the lake. The average productivity values

eporxd by Nair et al. (1983) was dill lower (143mg Um3/hour) for the Ashtamudilake. There seems to be an enhaced primary production activity during the postbarragephase consequent to a more stable and richer nutrient level at this sector,triggered by the stagnancy during the post-monsoon season.The benthic fauna was composed of 10 groups, mainly dominated by Amphipoda,Polychaeta and Oligochaeta at the 13 stations. The post monsoon period was themost favourable period for the incidence of benthic fauna. The low salineconditions coupled with hish dissolved oxygen levels and presence of largequantities of plant matter (macrophytic biomass) could be the reasons for the highincidence and abundance of amphipoda during the present investigation. Thenature of the sediment was aq important factor that influenced the distributions ofthe benthic organisms. The high organic carbon content in combination withhgiher percentage of sandy clay during the post monsoon and pre monson periodscould be one of the contlibuting factors for the rich ebnthic faunal diversity.Ten species of polychaetes namely Dendronereis sp., D, aestruarina, D. arborifera,Nereis chingrihanensis, Ceratonereis, rnirabilis, Ceratonereis sp. Prionospiocirrilera, Prionospio sp. and Capitellid sp, were identified during the period 1996-97 from the 12 stations. Polychaetes contributed 35% of the total benthic fauna inthe present study. The diversity index (HI, and richness index (d), evennes index(e) and dom~nance index (d) for the polychaete species were computed. Thehigher divers~ty of the polychaetes observed at the 12 stations during the studycould be due to theri euryhaline nature, when compared to the reference station(13) where only Ceratonereis sp and Gendronereis sp. occured. Villoritacyprinoides formed the bivalve population whereas Melanoides tuberct~latus,Melarnpus sp., lndoplanorbis exustus and Viviparus sp. represented the gastropodpopulation during the study. Earlier studies conducted by Sabastidn (1976) andKurup et a/. (1990) in the same study area reported the presence of bivalves,V.cyprinoides, M. meretrix and M. cas~a. The low saline nature of the bottomwaters during the present study showed the presence of only one species of V.cyprinoides. The low values of diversity index (HI, richness index (d) andevenness index (e) of the fauna were notable aspect of the study.

Pesticide residua in water, #dimcnt md fwnr d V mhnd lakeThe pesticide residue (BHC, Endosulphan) levels in water, sediment, and V.cyprinoides were detected during the post mwoon period 1995. The B. H. C.level was detected in water, which varied from 0.048 to 0.152 pg/l and that insediment from 0.0006 to 0.01 27pg/g. The clam had endosulfan in their tissws @0.366 pg/g. The concentration levels were within the safe limits prescribed forfishery waters.Characteristics of the major groups of benthic faunaN m u : Thew m P~O~O)CII wms or ntmrt~mm belonyng to (tu phylum Nmrtlnea Tho IS on oftlw r.~rrn group of benth~c laund observed from lhe Vembanad lake and th~s IS the f~nl account of nemenlne,from the bdckwatm Thee bnhc group* were prmnl In all warn, showlog In maxlrnurn man blornan Intlw monsoon I39 ndm'l fdlowd by the pre monsoon 132 dm') and thc pat m o m (7 ndrn') Stateon 1ruclvlnm a heavy dcM d agncukurdl dnd @her domert~c warn showed the h~.qhesl numer~cal abundance ofthe fduna (54 ndrn'l whrredr stdtlon 1 l showed thr lower1 12 ndrn'lFamilySpecies1. Tubificidae Auloddlus sp.Aulodrilus remexLimnodrilus hoffmeisteri2. Nereidae Namalycastis indicaDendronereis arborifere3. Naididae Dero sp.Dero coopenHanochaeta sp.

lhJm md tabqodr Onty m spxlcr m6emd by Vlllonu rwrmrdn lmnsd fhr bbvalvr popllam w h nmne $en nmvly Mdnodu Idmdau~. Mdmpm sp. Indspknortur mntu* and V r n w (onrd ttnRwrclpaf popvlnm (TIMCI The pcmanmn pnod dnmd Ihr h&rsl wdm d tnvah*, nd prfmpdr Abut#Is% d the wrrmpod capulnm wa mnt- by spxm Mdanonln tvbmulaur *. Indoplrwxb~~ cxu,turro I I %I howd hr rntn!mum pmn y mdcm The udv clrr nature d liv wbslmwn wllh h~@t un(ma c~ttxmlmtmt rt lotmr 1.3 and 4 d be IIW maa, hw Ihr hr b w r nd d~wnnry b ttn frunr In fhr ahrr unlm, thrrandy "dun d rtr Mom nd mnprraNtly law qanc -1 h w d h&wr lrunrl ~nrdrmr mnd rbunddncvFish and FisheriesTwenty six landing centres were identified from around the fresh water sector ofthe lake and data on the total landings of fish and prawn were collected during thestudy period from these areas. About 38 species of fishes and 4 species of prawnswere identified from the study (Table 7). The major fish species recorded from theinvestigation were Erroplus suratensis, f. rnaculatus, Channa rnarulius, Lahodussumieri, Puntius sp. Lutianus argentirnaculatus, Mystus sp., Tachysurus sp. andHernirarnphus sp.The fishery of M. rosenbergii, which was to the extent of 305 tonnes during 1960-61 (Raman, 1965) in the lake, was reduced to about 39tonnes by 1987-88 (Kurup,1989). This reduction is attributed to the barrage as it prevents the returnmigration of the spent brooders and the upward migration of the post larvae to theriven. The barrage has posed an obstacle to them during their peak migration InNovember to February. The fishery has, of late, shown some recoven/ due to thereduction the closure season from 6 to 4 months, and also due to the damagedcondition of some of the sluices.

There was a substantial change in the faunal canposition whm compared to thepre- and pt-barrage phases. The impobant faunal changes are indicatd in Table8.Ilpuntius uranaP ~r~amentosusI P.rt~rhecoki l d h duss~mirri! l a h rohiu! c>tl/d cat14! 11.l~&dylum1 Wallago anu: Ompk bimaculatusI Amblypharyngbdon molai t iec~ropncustes fossilir, < larrur batrachur: Marlacrmbelus armatusI Macrognalhus guintheri1 Channa striatusL Anabas tcuudi~usQuarter-w~se y~eld of E. surdtensis and M. msenbergii

A view of the endangered fish species fromVembanad Lake.

Tabk.7 Fish hndinlp holn Vembmd (South of thc b.n.gr)~ p c ~ ~ r o u ~ r 1995% 1-97(Kg.) g. (Kg.) kEtroplus surarcnsis 136 895 46.9 202 040 52.77Mullus 4000 1.3 4 450 1.16Channa suiatus 4 935 1.7 5912 1.54Channa rnarulius 10 949 3.8 14 828 3.87labeo dussumieri 6 087 2.1 23 742 6.20Labe0 rohita 472 0.2 586 0.15Cvprinur carpio 32 0.01Puntius spp. 31 586 8.15lutianus argentimaculatus 6 491 2.2 4 212 1.10iates calcarifer 2 137 0.7 1 822 0.47Scatophugus argus 3 852 1 .O1Anabas testudineus 1412 0.37Mystus sp. 14 767 5.1 5 008 1.31Horabagrus brachysoma 2 150 0.56Techvsurus arius 11 381 2.97Hemiramphus sp. 18 389 6.3 25 578 6.68Chams chanos 1 739 0.6 1 370 0.36Omer speciesT0I.l fkh

Urcrobrachium rosenbergir 57 930 47.0 36 332 35.55U ~&lb 27 002 21.9 33 250 32.53Melapenaeur dobsonii 36 713 29.8 31 430 30.75H. ~ O~KKC~OI 1 067 0.9 1188 1.16ToQl prawn 123 350 29.7 102 200 21.1TOTAL LANDINGS: 41 5 303 485 041Excld~np cr&& molluus)

A busy scene at one of the landing sites.The landings -The species comprise of brackishwater,freshwater and marine migrant forms.

The study reveals that the Thanneennukkom barrier commissioned in 1975 across thelake has consi&rably altered the ecology and fishery resources 01 the lake sector south 01the barrage. The mapr changes observed during the post-barrage phase in comparison tothat of the p&anage phase were the following:A marked reduction m the depth due to the accumulation of the silt lrom the incomingrrvers, bringrng down the water holding capacity of the lake.The trdal mfluence reduced from 80 km upstream to 40 km till the barrage dunng thesummerThe Increase m weed growth and a severe restrtctron on the natural flushrng of thepollutants enterrng the sector dunng the summer months when the barrage nmarnrclosedChanges rn spatla1 and temporal varlatrons rn the water and redrment qualrtv - a \tt.rpreductron rn the salrnrty, an elevatron of nutrrent level in the water phase ar~d dl1elevatron of the organlc carbon m the sedrment The changes were more pronounred rnthe southern end of the sector, where an eutrophrcatron tendency exrsted, geared hv thestagnancy consequent to the closure of the barrage durrng December to AprrlChanges rn the composrtion of the plankton and benthic organisms, the salrnc. spec.rtAslargely grvrng way to euryhalrne or freshwater species.An enhancement m the prrmary productron levels at the south end of the scat tor clurrn~the stagnant monthsThe reductron rn the upstream and downstream migration of fish and prawns. The stretrhof the lake above the barrage becoming inaccessible to the euryhalrne mrgratory frsherand prawns lrom the sea, the fish productivity of the sector dwrndled About 13 sucheuryhalrne migratory specres no more contribute to the fishew of this sector. Srntrlarlyabout 16 stenohalrne freshwater species found an entry to the commercral fishery of theSector The pnzed fishery of Macrobrachiurn rosenbergii got reduced to about 35% of rtslandings durrng the pre-barrage phase (196365).

The study indicates Ihe necessity of maintaining the continuity of the lake berween bothsides of the barrage so that the tidal washings during the p msoon months enabks tonaturally llush out the wades and thefeby mkKing ttte 1ves5 on the environment at thisSector.The continuity would also ensure the upward and downwad migration of the lish andprawn any time during the year making the whok lake available to the euryhalinespecies. This would ~ubstantially improw the fishery of the lake as a whole. The upwardand downward migration of the Macrobrachiurn specter also would be facilitated lead~ngto the rejuvenation of its fishery. The changes w l d haw a salutary effect on the socioeconomiccondition 01 the lisherfolk engaged in fishery 01 the lake.It is tberelore suggested that there should be only periodic closure of barrage rather thanpermanently remained closed for about 4-5 months. The periodicity should depend onthe tidal amplitude so that the'second crop olpaddy cuhivation is not adversely affecteddue to the salinity intrusion, and the tidal amplitude does not reach the intake polnt ofdrinking water lor the Kottayam town in Meenachal river.

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